Radiation therapy is usually defined as the use of high-energy radiation from x-rays, gamma rays, neutrons, protons, and other sources to kill cancer cells and shrink tumors. Radiation may come from a machine outside the body also called external-beam radiation therapy, or it may come from radioactive material placed in the body near cancer cells also called internal radiation therapy.
Radiation therapy, however, has its limitations. The ionizing radiation used to ablate unwanted tissue can cause damage to surrounding healthy tissue, or may not be effective against the target tissue due to conditions such as hypoxia which makes the targeted cells radioresistent or cells being in a part of the mitotic cycle where they are not as sensitive to the effects of such radiation. Much study and effort has been expended developing compounds and techniques to enhance the effectiveness of radiation therapy and limit the damage to healthy, non-targeted tissue.
Radiosensitizers are drugs created to enhance the effectiveness of radiation therapy by making tumorigenic cells more susceptible to the effects of radiation. One class of radiosensitezers, known as halogenated pyrimidines, accomplishes this enhancing effect by directly making DNA more susceptible to damage from radiation. This class of radiosensitizers works by incorporating halogenated pyrimidines directly into the DNA chain in substitution of thymidine. This substitution weakens the DNA chain and makes cells more susceptible to radiation and ultraviolet light. Another class of radiosensitizers functions by fast ionization/deexcitation processes and the strong emission of secondary electrons. Yet another class of radiosensitizers, known as hypoxic-cell sensitizers, increase the radiation sensitivity of tumorigenic cells deficient in molecular oxygen.
The radiosensitizing effects of these drugs are believed to aid the ionizing radiation by augmenting the latter's ability to damage nuclear DNA in creating strand breaks that are not repairable, therefore triggering apoptosis. It has also been theorized that the drug cisplatin, a chemotherapeutic drug that is known to have radiosensitizing properties, may cause damage to mitochondrial structures.
Further, cellular respiration is the set of metabolic processes by which biochemical energy from nutrients is converted to energy in the form of adenosine triphosphate (“ATP”). During normal aerobic cellular respiration, one molecule of glucose, the most abundant nutrient in mammalian serum, is converted to two molecules of pyruvate and two net molecules of ATP. This process is known as glycolysis. The pyruvate is then further broken down in order to release a theoretical yield of 36-38 molecules of ATP.
The mitochondria, which play an important part in the aerobic cellular respiration process, are spherical or elongated organelle in the cytoplasm of nearly all eukaryotic cells, containing genetic material and many enzymes important for cell metabolism, including those responsible for the conversion of food to usable energy. Mitochondria provide the energy a cell needs to move, divide, produce secretory products, contract—in short, they are the power centers of the cell. They are about the size of bacteria but may have different shapes depending on the cell type.
Mitochondria are membrane-bound organelles, and like the nucleus have a double membrane. The outer membrane is fairly smooth; the inner membrane is highly convoluted, forming folds called cristae. The cristae greatly increase the inner membrane's surface area, and it is here that mitochondrial electron transport occurs.
The elaborate structure of a mitochondrion is very important to the functioning of the organelle. Two specialized membranes encircle each mitochondrion present in a cell, dividing the organelle into a narrow inter-membrane space and a much larger internal matrix, each of which contains highly specialized proteins. The outer membrane of a mitochondrion contains many channels formed by the protein porin and acts like a sieve, filtering out molecules that are too big. Similarly, the inner membrane, which is highly convoluted so that a large number of in-foldings called cristae are formed, also allows only certain molecules to pass through it and is much more selective than the outer membrane. To make certain that only those materials essential to the matrix are allowed into it, the inner membrane utilizes a group of transport proteins that will only transport the correct molecules. Together, the various compartments of a mitochondrion are able to work in harmony to generate ATP in a complex multi-step process.
The mitochondrion is different from most other organelles because it has its own circular DNA (similar to the DNA of prokaryotes) and reproduces independently of the cell in which it is found; an apparent case of endosymbiosis. Mitochondrial DNA is localized to the matrix, which also contains a host of enzymes, as well as ribosomes for protein synthesis. Many of the critical metabolic steps of cellular respiration are catalyzed by enzymes that are able to diffuse through the mitochondrial matrix. The other proteins involved in respiration, including the enzyme that generates ATP, are embedded within the mitochondrial inner membrane. In-folding of the cristae dramatically increases the surface area available for hosting the enzymes responsible for cellular respiration.
Human mitochondria contain 5 to 10 identical, circular molecules of DNA. Each molecule contains 16,569 base pairs that encode 37 genes including ribosomal RNA (rRNA), transfer RNA (tRNA), and 13 polypeptides. The 13 proteins are an important part of the protein complexes in the inner mitochondrial membrane, forming part of complexes I, III, IV, and V. These protein complexes also dependent upon proteins encoded by nuclear DNA, which are synthesized in the cytosol and imported into the mitochondria.
In the absence oxygen, a hypoxic cell can still generate energy through glycolysis and generate two net molecules of ATP. However, under such hypoxic conditions, the resulting pyruvate is not transported into the mitochondria for further processing, but rather remains in the cytoplasm where it is converted to lactate by lactic acid fermentation and expelled from the cell. This process is known as anaerobic respiration.
Interestingly, it has been observed for some time that even in the presence of oxygen, rapidly proliferating tumorigenic cells have a preference for inefficient anaerobic respiration and therefore utilize an abnormally high amount of glucose. This is known as aerobic glycolysis, or the Warburg Effect, named after Otto Heinrich Warburg, who made the discovery in 1926. Various theories have been put forth to account for this effect, among which is that glucose degradation provides cells with intermediaries used in a variety of biosynthetic pathways. It is therefore theorized that tumor cells maintain robust glycolysis in order to keep a ready supply of such intermediaries.
Glucose is not however, the only compound to be consumed at highly elevated levels by proliferating cancerous cells. These cells also use copious amounts of glutamine relative to non-tumorigenic cells. Glutamine is a non-essential amino acid present abundantly throughout the body and is involved in many metabolic processes. It is synthesized from glutamic acid and ammonia. It is the principal carrier of nitrogen in the body and is an important energy source for many cells.
In cancerous cells, the TCA cycle is truncated because such cells use carbon from the cycle for biosynthetic purposes. Citrate therefore is unlikely to cycle all the way back around and regenerate oxaloacetic acid (“OAA”). Tumors solve the problem of the need to regenerate OAA—and also generate much of the energy they need to proliferate—by oxidizing large amounts of the amino acid glutamine and incorporating it into the truncated TCA cycle. In tumorigenic cells, the truncated TCA cycle incorporates glutamine and pyruvate supplied by the phosphorylation of glucose to generate energy and create precursors for biosynthetic pathways. The phenomenon of significantly increased glutamine utilization in tumorigenic cells has been previously studied as a potential pathway by which therapeutic anti-cancer drugs may act. The glutamine analogues L-[alpha S,5S]-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (acivicin) and 6-diazo-5-oxo-L-norleucine (DON) are known to possess cytotoxic activity against a wide variety of tumors. These drugs are thought to function by inhibiting mitochondrial enzymatic activity. However, their usefulness as therapies for humans has been limited due to their high toxicity.
To date, there has been no drug specifically designed as a radiosensitizer that targets the mitochondria of tumorigenic tissue and cells for destruction.
Physical Aspects of High-Z Materials and Charged Particle Amplification
The following discusses the effects of high-Z materials on the atomic (picometer, or 1E-12 meter) scale, by comparing the individual interaction rates of different materials when exposed to a fluence of charged particles and gamma- or x-ray photons. Furthermore, in the context of therapeutic radiation, the energy deposition models provided herein for the orthovoltage and megavoltage energy ranges are 2E5 to 1.8E7 eV, or 3.2E-14 to 2.88E-12 J.
For therapeutic irradiation, tissue is exposed to a calibrated beam of electrons or photons. Photons indirectly interact with matter through coherent, photoelectric, Compton, or pair production collisions. Coherent scattering results in no energy deposition, and will not be discussed further. The remaining collisions result in the emission or ejection of electrons. The scattered electrons further deposit energy by directly interacting with nearby atoms in collisional or radiative type events, potentially ejecting additional electrons (6 rays). The total amount of kinetic energy per unit mass lost from the photons and 6 (delta) rays in non-radiative processes is referred to as collision kerma, or Kc. The units are typically given in J/kg, or Gy. In the presence of charged particle equilibrium, the total amount of absorbed dose is equal to the collision kerma. In surrounding matter, the dose deposition process results in the generation of free electrons and ions which can damage the DNA or other cellular structures; in the case of the present invention, the mitochondria. Collision kerma can be calculated directly from the collision probabilities (or cross sections) of each interaction using the formula as in FIG. 1, where ψ (psi) refers to the incident photon energy fluence in J/cm2, P is the material density in g/cm3, g is the average fraction of secondary electron energy lost to radiative processes. The values Ttr, σtr, and Ktr, refer to the energy transfer cross sections, in cm−1, for photoelectric, Compton, and pair production interactions, respectively.
For incident photon energies of 0.5 to 5 MeV on almost all materials, the Compton cross section dominates the above equation; that is, σtr>Ttr, Ktr. The cross section for Compton interactions has been rigorously modeled by Klein and Nishina (Evans, 1955), as shown in FIG. 2, who defined the following statement for σtr, where r0 refers to the classical electron radius e2/m0c2=2.818×10−13 cm, NA=6.022×1023 mole−1 is Avagadro's constant, Z is the number of electrons per atom, Aw, is the atomic weight in grams, h=6.626×10−34 is Planck's constant in J-s, v is the frequency of incident radiation in cm-1, m0=0.91095×10−30 kg is the rest mass of an electron, and c=2.9979×1010 cm/sec is the speed of light.
For incident electrons with energy T (in J), the expectation value for rate of energy loss due to collisional events through a linear distance x (in cm) can be described by the collision stopping power of a material, or (dT/dx)c. FIG. 3 defines the collision stopping power for electrons adjusted for the polarization effect and shell correction, where l is the mean ionization/excitation potential (Berger & Seltzer, 1983) of the material in J, δ is the polarization correction parameter (Stermheimer, 1952), and c is the shell correction parameter (Bichsel, 1968).
Similarly, the expectation value for energy loss due to radiative events, i.e. bremsstrahlung, is described by the radiative stopping power (dT/dX)r, and is shown in FIG. 4. The value Br is defined by Bethe and Heitler (Evans, 1955), and carries a slight dependence on Z and T.
The radiation yield, therefore, is simply the mean ratio of energy loss to radiative processes relative to the total rate of energy loss over all initial electron energies and as each electron loses energy. FIG. 5 shows the radiation yield formula, where Tmax refers to the maximum initial electron energy.
In order to achieve an increased dosimetric effect from external ionizing radiation, targeted molecules located around a biological target can be replaced with appropriate analogues that contain one or more high-Z elements. An important quantifier for this effect can be defined as the relative increase in the expectation value of charged particle fluence created by the high-Z analogue over that of the original molecule. This value, herein referred to as the amount of charged particle amplification A, is shown in FIG. 6.
As defined above, the value of A is dependent on the type of molecule used for high-Z implementation. Furthermore, the effects of molecular binding on each high-Z atom will modify slightly the above equations that define the interaction rates. That said, numerical values for A can be estimated and quantified for each individual high-Z elemental substitution performed in a molecule using the above formulas. For photon interactions, the increase in charged particle fluence is simply the ratio of energy transfer interaction probabilities (the subscript a is used to denote these probabilities in units of cm2/atom). Similarly, the reduction in fluence may be estimated for electron interactions by comparing the ratio of energy lost to radiative processes. FIG. 7 presents a formula wherein these results compete to formulate A, where the superscript z refers to the interaction cross sections for the high-Z material, while c refers to the element substituted (considered a carbon atom).
Values for A have been plotted in FIG. 8 for atomic numbers Z=1 through 90, where carbon (Z=6) is used as the reference, using published energy absorption cross sections (Seltzer, 1993). Data for three incident photon energies has been given: a monoenergetic 500 keV theoretical beam, and polyenergetic 6 MV and 18 MV beams typically found on a modern therapeutic linear accelerator. As an example, three gold atoms (Z=79) would yield a factor of 17×3=51:1 higher fluence rate than three carbon atoms present in the same molecule for a 6 MV photon beam. The same replacement would yield a factor of 136:1 and 82:1 for 500 keV photons and an 18 MV beam, respectively. In order to apply this value to the entire molecule, A can be expanded to include the atomic fractions f1, f2 of each element with atomic number Z1, Z2, etc. present in the compound. In addition, Bragg's rule can be applied to estimate the mean ionization potential and polarization correction, as shown in FIG. 9.